When such a fuel oil as described above is exposed to low temperatures, the wax component of the oil precipitates as crystals. Thus, if it is used as a fuel for a Diesel engine, it causes filter plugging. This causes poor operation of the engine, or in an extreme case, the oil solidifies within the fuel pipe and completely fails to flow.
In order to solve this problem, various wax crystal modifying agents (improvers) which interact with the wax crystals have previously been proposed. As such crystal modifying agents, Japanese Patent Publication No. 23165/73, for example, teaches the use of an ethylene copolymer having a number average molecular weight of 1,000 to 2,900 and containing 2.4 to 25 mol % of an ethylenically unsaturated ester and not more than 6 alkyl branches per 100 methylene groups. Japanese Patent Application Laid-Open No. 141390/81, for example, teaches the use of an ethylene copolymer having a number average molecular weight of 1,000 to 5,000 and containing 10 to 50% by weight of an ethylenically unsaturated ester and 6 to 15 alkyl branches per 100 methylene groups. These proposed modifying agents can effectively improve plugging resistance and flowability of middle distillate fuel oils with the difference between the 20% distillation temperature and the 90% distillation temperature being as broad as at least 100.degree. C., but have no improving effect on the plugging resistance of middle distillate fuel oils with the above temperature difference being as relatively narrow as less than 100.degree. C. although they can also improve the flowability of the oils having the relatively narrow distillation temperature difference.
Japanese Patent Publication No. 20069/64 teaches, for example, an ethylene copolymer having a number average molecular weight of 700 to 3,000 and containing 1 to 40% by weight of a vinyl ester. This copolymer can effectively act on middle distillate fuel oils having a narrow distillation temperature difference only by regulating its alkyl branches.
A typical example of the middle distillate fuel oils having a narrow distillation temperature difference is a light oil. To improve the plugging resistance of the light oil, it is the general practice to mix it with a kerosene fraction which has a lower boiling point. Such a method, however, is economically disadvatageous, and also leads to a reduction in the ratio of obtaining kerosene. Hence, it is not industrially advantageous.